1. Field of the Invention
The invention relates to techniques for surface deionization of glass by movement of ions under the effect of an electrical field and, in particular, to the dealkalization of soda-lime glass.
2. Discussion of the Background
Dealkalization treatments are conventionally carried out to modify the chemical resistance, improve the fixing of surface deposits or modify certain optical characteristics of the glass.
The most frequently used deionization is of the chemical type. The glass object is subjected to a solution or an atmosphere which reacts with the alkaline ions of the glass. To facilitate the reaction, the operation takes place at a temperature which is greater than room temperature.
More recently techniques have appeared in which the surface deionization is carried out using ion migration under the effect of an electrical field.
In principle, this phenomenon is as follows. The glass, whose temperature is controlled, is placed between two electrodes. The application of the electrical field mobilizes the more easily movable ions and, in particular, the alkaline ions. Under the effect of a continuous field, the alkaline ions are moved towards the cathode and form a carbonate skin on the surface of the glass and in contact with the atmosphere. Oxygen and/or hydrogen diffuse on the surface which is exposed to the anode.
From the practical view point, it is very difficult to obtain a homogeneous treatment of the surface of the glass. The application of the field using solid electrodes leads to the formation of local spots. In order to obtain a uniform treatment, it has been suggested to subject the glass to "gaseous electrodes", preferably in a rarified atmosphere and with relatively high applied voltages, on the order of several thousand volts.
This method is particularly delicate. Due to the high voltage, there are high risks of arc formation. Also, this technique has only been proposed for actual use on small-sized specimens and under conditions which provide perfect control of the respective positions of the glass and the electrodes as well as of the atmosphere in which it is carried out. Furthermore, in accordance with this technique, the temperatures in which the operation was proposed were relatively low.